How the floor calculation works
When a floor sits over unheated space, the floor itself is the barrier between inside and outside, so it gets its own R-value target. The calculator uses ENERGY STAR's recommended levels, which follow the 2021 IECC:
| Climate zone | Floor target |
|---|---|
| 1–2 | R-13 |
| 3–4 (except 4C) | R-19 |
| 4C, 5–6 | R-30 |
| 7–8 | R-38 |
The steps are the same as for an attic:
- Measure the floor above the crawl space, garage or unheated basement. Length × width of each section, added up.
- Enter any existing insulation by depth and type, or by R-value.
- Enter your ZIP code to get your target.
- Choose a product. The calculator picks the fewest batt layers that reach the target, then the thinnest such stack, and counts bags from the coverage printed on each one.
Floor insulation goes between the joists, so think about joist depth before you buy. The calculator doesn't limit depth for floors, so compare its answer with your joists: 2×8s are 7.25 in deep, 2×10s are 9.25 in and 2×12s are 11.25 in.
Worked example: 900 sq ft over a crawl space in zone 5A
A 900 sq ft floor over a vented crawl space, no existing insulation, in climate zone 5A:
- Target: R-30
- Area with 10% waste: 990 sq ft
| Product | Layer | To buy | Material cost |
|---|---|---|---|
| Fiberglass batts, 24 in wide | R-30, 9.5 in | 12 bags | $1,560 |
| Mineral wool batts | R-30, 7.25 in | 34 bags | $3,297 |
| Closed-cell spray foam | 4.62 in | Installed by a pro | $6,231–$10,385 |
The R-30 fiberglass batt suits 2×10 joists. The mineral wool batt is thinner for the same R and fits 2×8 joists exactly, but in this example it costs about twice as much.
The same 900 sq ft floor in other zones:
- Zone 3A (R-19): one layer of R-19 fiberglass, 6.25 in, 9 bags, $1,071
- Zone 7 (R-38): one layer of R-38 fiberglass, 12 in, 16 bags, $1,872–$2,064
An R-38 batt at 12 in is deeper than a 2×10 or 2×12 joist bay. In zones 7 and 8, check that the batt can go in without being compressed.
Installing floor batts
Facing goes toward the heated space
If you use faced batts, the facing goes up against the subfloor, toward the warm side. Unfaced batts are also an option. Vapor retarder rules vary by climate, so check your local code.
Keep the batt against the subfloor
A gap between the batt and the floor above lets cold air circulate over the top of the insulation. Push each batt up until it touches the subfloor, then hold it there with:
- spring-steel insulation supports pressed between the joists,
- wire laced across the joists and stapled, or
- wire or plastic mesh stapled to the joist undersides.
Don't pull supports so tight that they compress the batt. A squashed batt loses part of its R-value.
Seal first
Before the batts go in, seal gaps where pipes, wires and ducts pass through the floor. Insulate the rim joist around the edge of the crawl space or basement too, because that's where the floor framing meets the outside.
Pipes and ducts
Once the floor is insulated, the crawl space below gets colder. Insulate water pipes and any ducts that run there, especially in cold zones.
Moisture comes first
Don't insulate over a damp crawl space. Fix drainage and standing water first, and cover a dirt floor with a ground vapor barrier. Wet insulation loses R-value and can grow mold. If the crawl space is sealed and conditioned rather than vented, the walls get insulated instead of the floor: use the basement insulation calculator for those walls.
Floors over garages
Rooms over a garage need the same floor R-value as rooms over a crawl space. Seal every gap in the garage ceiling first so car exhaust and fumes can't reach the room above. Fill the joist bays fully, because an air gap above the batts lets cold air circulate right under the floor.
Check your zone's numbers on insulation R-values by climate zone and compare batt depths on the R-value chart. For the attic above, use the attic insulation calculator.